Like Amma said you could just back it and quilt it. I do think tho as Great Aunt Jacqui said flannel would be good to. It would give it just a bit of umph so to speak and not be heavy but would stabilize the quilt more. But also as TonniLoree said you could just add a blanket for the batting.
My suggestion would be to clarify with your granddaughter exactly the look/feel/weight she was was desiring...sometimes we say one thing but actually mean something else entirely.